Radium Tracing Cross‐Shelf Fluxes of Nutrients in the Northwest Pacific Ocean

• Published in: Geophysical research letters Vol. 46; no. 20; pp. 11321 - 11328
• Main Authors: Cho, Hyung‐Mi, Kim, Guebuem, Kwon, Eun Young, Han, Yongjin
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28-10-2019
• Subjects: Anthropogenic factors; Atmosphere; Atmospheric models; Atmospheric pollution deposition; Biogeochemical cycle; Biogeochemical cycles; Biogeochemistry; Biological production; Carbon sequestration; continental shelf; Continental shelves; flux; Fluxes; Groundwater; Human influences; Mineral nutrients; North Pacific; Nutrient cycles; Nutrients; Oceans; Phosphorus; Radium; Ratios; River water; Rivers; Surface water; Tracers; Typhoons; Water circulation; Water column; East China Sea; Yellow Sea; Pacific Ocean
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2019GL084594

Nutrients in surface waters of the northwest Pacific Ocean are known to be influenced significantly by anthropogenic inputs via the atmosphere. In this study, we evaluated the relative contribution of cross‐shelf fluxes in the northwest Pacific Ocean. The shelf fluxes were estimated by multiplying the 228Ra flux based on inverse modeling by the measured ratios of nutrients to 228Ra. The ratios were obtained from field observations of nutrients and 228Ra in the Yellow Sea and the East China Sea in February 2017 when the water column was fully mixed vertically. The cross‐shelf fluxes are estimated to be approximately 40% of the atmospheric depositional flux of nitrogen and 2 orders of magnitude greater than that of phosphorus. These results suggest that the cross‐shelf fluxes of nutrients are an important, yet previously underappreciated, pathway of nutrients to the North Pacific Ocean, controlling carbon sequestration and biological production.
Plain Language Summary
Changes in the nutrients and biogeochemical cycles in the surface waters of the northwest Pacific Ocean have been mainly attributed to the atmospheric inputs of anthropogenic nutrients. However, we hypothesized that continental shelf waters, which receive large inputs of nutrients from rivers, groundwater, and the atmosphere, can deliver significant amounts of nutrients to the northwest Pacific Ocean. The relative importance of atmospheric fluxes versus cross‐shelf fluxes is particularly important in this region since (1) the Yellow Sea and the East China Sea continental shelf is one of the largest continental shelves in the world, (2) Changjiang River water is highly enriched in anthropogenic nutrients, and (3) the atmospheric deposition of nutrients in the northwest Pacific continental shelves is known to be the largest in the world. Our new approach using 228Ra as a tracer allows us to estimate cross‐shelf fluxes of nutrients which are extremely difficult with direct measurements. The results show that cross‐shelf fluxes of nutrients are as important as atmospheric inputs.
Key Points
The cross‐shelf fluxes of nutrients are determined by multiplying measured nutrient/228Ra ratios by the model‐driven 228Ra flux
The flux of dissolved N from the shelf water is 30%‐40% of its direct atmospheric depositional flux
The flux of dissolved P from the shelf water is 2 orders of magnitude greater than the atmospheric total P flux